Both 早速【さっそく】 and さっさと are adverbs with the meaning "immediately, quickly, at once." What distinction in connotation or usage exists between the two terms?
さっさと is kind of rude way or try to order someone else, when you try hurry someone else. I.e. さっさとやれよ. (Go do it quick man!)
さっそく is more often use as positive way which you think people do it quicker than you thought, such as immediately. I.e. さっそく返事してくれてありがとう。(Thank you for reply me immediately)